Over the past several years, PCM (pulse-code modulation) cable carrier systems have seen tremendous growth in exchange trunk transmission. In some locations multi-pair exchange cables have become fully utilized, necessitating the addition of more cables and more line repeaters. In this surrounding, digital multiplexers have been developed for combining the digital outputs of severaL PCM terminals into a single, higher-rate composite bit stream. See "Digital Multiplexers", Transmission Systems for Communications, Fourth Edition, Chapter 26, Bell Laboratories, Inc., 1970.
The typical approach to time-division multiplexing of the digital signals from several PCM terminals, as explained in Transmission Systems for Communications, supra, is to consider the several bit streams as synchronous digital signals. This technique provides maximum flexibility since it permits the digital signal sources to be separated by large distances, i.e., it does not require prior synchronization of the individual bit streams. If the multiplexer input signals are asynchronous, then elastic stores must be used to periodically delay the incoming bit streams. Digital storage provided by elastic stores permits pulse stuffing to be used to increase the bit rates of all incoming signals to that of a local clock signal. Variable pulse stuffing, provided by elastic stores, permits maximum system flexibility since few constraints are placed on the PCM terminal output signals. For a full discussion of pulse stuffing, see V. I. Johannes and R. H. McCullough, "Multiplexing of Asynchronous Digital Signals Using Pulse Stuffing with Added Bit Signaling", IEEE Transactions on Communication Technology, October 1966, pp. 562 - 568; and U.S. Pat. No. 2,548,661.
Elastic stores and their control circuitry are unfortunately rather complex and expensive circuits, and their cost necessarily increases the overall system cost of a digital multiplexing system. Also, variable pulse stuffing requires an added information channel to inform the receive terminal of the location of the stuffed pulses. Although such information is usually sent on a time-share basis with the framing bits, it is still wasteful in the sense of requiring an added communication channel.
In contrast, the invention described herein does not require the use of elastic stores in a digital multiplexing system of the type just described. Further, the system herein makes full use of its signaling channel since only framing bits need be sent to the receive terminal. The only added requirement placed on the use of the system herein is that the digital sources be placed in close proximity so that the bit streams can be bit synchronized to the same clock signal before entering the digital multiplexer.
It is an object of this invention to provide a digital multiplexing system design technique capable of multiplexing N bit-synchronized digital signals, each at a bit rate f.sub.1 into a digital composite line signal having a bit rate f.sub.2, where Nf.sub.1 &lt; f.sub.2.
It is a further object of this invention to provide a digital demultiplexer not requiring elastic stores.